A significant advantage of a bolted joint over other fastener types, such as welded and riveted joints, is that such bolted joints are capable of being disconnected. This can, however, cause problems if loosening and/or subsequent disconnection unintentionally occurs as a result of operational conditions. Such unintentional loosening, often called vibrational loosening, may be caused by the side movement of the nut or bolt head relative to the joint, resulting in relative motion occurring in the threads of the fastener. Such joints are acceptable for use in equipment where the joint is not undergoing high vibrations. However, problems may occur when the joint is subject to high vibrations and/or when high dynamic loads are applied. During vibrations, localised slip at the bolt/ nut contact surfaces can loosen threaded fasteners. Friction-based, spring-based and interference-fit fasteners do not handle these conditions well because of their natural tendency to follow the threads and make the bolt move away from the joint. A nut has a preferred direction of rotation when subjected to vibrations and thread friction has been overcome. Only a small amount of vibration-induced fastener movement can significantly reduce the clamp load and once some clamp load is lost and the bolt moves even more, further clearance is created in the shaft and the weakening bolts can shear due to wear and tear.
Thus, pre-loaded bolts or nuts may rotate and become loose due to relative motion occurring between the male and female threads of the fastener. Three common causes of relative motion are: (1) bending of parts which results in the forces being induced at the friction surface. If slip occurs, the head and the threads will slip which may lead to loosening; (2) differential thermal effects caused as a result of either differences in temperature or differences in clamped materials; and/or (3) applied forces on the joint which can lead to shifting of the joint surfaces leading to bolt and/or nut loosening. In general, the key to preventing self loosening of fasteners is to ensure that, for example, there is sufficient clamp force present on the joint interface to prevent relative motion between the bolt head or nut and the joint; and that the joint is designed to allow for the effects of embedding and stress relaxation.
Rotatable captive nuts have been used on equipment where access is required for servicing and/or electrical disconnection is required for safety. The provision of a captive nut reduces the possibility that the nut may be lost when disconnected. In addition, the use of a captive nut reduces the number of components required during assembly of a system.
Rotatable captive nuts often comprise threaded fasteners. However, such threaded fasteners do not allow for rapid assembly and/or disassembly of a system such as a unit or other installation. In addition, during assembly of the system, the threaded fastener must be tightened to create a preload across the assembled joint such that energy is introduced in the form of tension, torsion and/or bending energy. This energy is held within the threaded fastener via friction produced by the mating of the male and female threads of the fastener and by the contact thrust faces of the nut. In the event that the frictional force is overcome, the energy that is stored in the fastener and the preload that has been generated will be lost. This may occur gradually over a period of time and may be caused by vibration. Depending on the nature of the vibrations and other conditions, there will be a finite resultant loosening torque that the fastener system has to resist. Therefore, if bolts are tightened to a particular preload in such a way that the remaining tension of the bolt under applied load is adequate to generate a friction torque larger than the loosening torque vibration, vibrational loosening will not occur. Without the preload, the fastener assembly will become loose and will eventually come apart as the male and female components of the threaded fastener disengage.
To gain enough thread friction to withstand vibrational loosening requires a higher torque and subsequent clamp load across the joint than is necessary to maintain the electrical contact. This higher torque requires a stronger structure around the stud head and mounting to withstand these torsional forces. Therefore, the supporting structure is over engineered. Assembly and reassembly after disassembly requires the use of speciality tools to ensure that the correct torque is applied. For example, such assembly and reassembly may require the use of a torque wrench. Torque is an unreliable method of tightening since it is influenced by many factors including lubrication, surface finish, thread surface damage and mating surfaces.
Also, when using a threaded fastener, a significant proportion of the clamp load that is provided across the fastened joint is required to maintain the friction across the threaded connection and to prevent the fastener from becoming loose.
In addition, thermal shock and the application of a load to a joint may cause the fastener to come undone. This is a particular problem when the threaded fastener is used with an electrical connection. In this case, a loose electrical connection may result in the passage of a high current through a system, resulting in the generation of heat which may result in fire or other heat damage.
It is an object of the invention to seek to mitigate problems such as the aforementioned.